package Others;

/** @author Prateek Kumar Oraon (https://github.com/prateekKrOraon) */
import java.util.Scanner;

// An implementaion of string matching using finite automata
public class StringMatchFiniteAutomata {

  public static final int CHARS = 256;
  public static int[][] FA;
  public static Scanner scanner = null;

  public static void main(String[] args) {

    scanner = new Scanner(System.in);
    System.out.println("Enter String");
    String text = scanner.nextLine();
    System.out.println("Enter pattern");
    String pat = scanner.nextLine();

    searchPat(text, pat);

    scanner.close();
  }

  public static void searchPat(String text, String pat) {

    int m = pat.length();
    int n = text.length();

    FA = new int[m + 1][CHARS];

    computeFA(pat, m, FA);

    int state = 0;
    for (int i = 0; i < n; i++) {
      state = FA[state][text.charAt(i)];

      if (state == m) {
        System.out.println("Pattern found at index " + (i - m + 1));
      }
    }
  }

  // Computes finite automata for the partern
  public static void computeFA(String pat, int m, int[][] FA) {

    for (int state = 0; state <= m; ++state) {
      for (int x = 0; x < CHARS; ++x) {
        FA[state][x] = getNextState(pat, m, state, x);
      }
    }
  }

  public static int getNextState(String pat, int m, int state, int x) {

    // if current state is less than length of pattern
    // and input character of pattern matches the character in the alphabet
    // then automata goes to next state
    if (state < m && x == pat.charAt(state)) {
      return state + 1;
    }

    for (int ns = state; ns > 0; ns--) {

      if (pat.charAt(ns - 1) == x) {

        for (int i = 0; i < ns - 1; i++) {

          if (pat.charAt(i) != pat.charAt(state - ns + i + 1)) {
            break;
          }

          if (i == ns - 1) {
            return ns;
          }
        }
      }
    }

    return 0;
  }
}
